1. Field of the Invention
The present invention pertains to a process for reducing or eliminating foam in an acid gas scrubbing process stream by the addition of certain polyoxyethylene polyoxypropylene block copolymers. The present invention also includes acid gas scrubbing solutions containing these block copolymers.
2. Description of the Prior Art
Acid gas scrubbing processes are of considerable industrial importance. Most acid gas scrubbing processes involve contacting a gas stream with an aqueous alkaline solution and absorbing the acid gas, principally carbon dioxide and hydrogen sulfide, into the solution and regenerating the solution in a separate stage. The regenerated solution is then recycled to the absorption stage. One of the problems encountered in acid gas scrubbing processes is excess foaming, i.e., aqueous alkaline solutions have a natural inclination to foam, and the passing of a gas into a foamable solution is classic to the production of a foam. Consequently, resort to the use of antifoaming agents as additives to the aqueous acid gas scrubbing solutions is a common practice in the art.
There are many commercially available antifoaming agents. These antifoaming agents find a plurality of uses. However, the selection of an antifoaming agent in acid gas scrubbing processes is dependent upon several variables unique to acid gas scrubbing processes. The antifoaming agents of the present invention are particularly suited for acid gas scrubbing processes.
The following U.S. patents were considered in connection with the present invention:
______________________________________ U.S. Pat. Nos. ______________________________________ 2,176,441 3,848,057 2,608,462 3,862,243 2,674,619 3,943,227 2,712,978 4,002,721 2,946,652 4,042,528 3,071,433 4,094,957 3,275,403 4,100,257 3,642,430 4,101,633 ______________________________________
None of these patents disclose, teach or suggest the use of polyoxyethylene polyoxypropylene block copolymers as anti-foaming agents in acid gas scrubbing processes. Many of these patents teach acid gas scrubbing processes and the general use of conventional antifoaming agents in their respective processes. Of particular interest are U.S. Pat. Nos. 4,094,957; 4,100,257; and 4,101,633, in addition to U.S. Pat. Nos. 4,112,050; 4,112,051; and 4,112,052, the disclosures of which are incorporated herein by reference. These patents teach and disclose the use of sterically hindered amines, alone, or in combination with other acid gas scrubbing compositions in acid gas scrubbing processes. These patents point out that there are three principal types of acid gas processes, i.e., the aqueous amine process (amine and water constitute the acid gas scrubbing solution), the "hot potash" process (an inorganic alkaline material such as potassium carbonate activated by at least one amine) and the organic solvent process (an organic solvent such as sulfolane used in combination with an amine). Foaming problems are especially troublesome in the "hot potash" type processes such as described and claimed in U.S. Pat. No. 4,094,957 and U.S. Pat. No. 4,112,050.
The prior art has included disclosures of certain block copolymers of oxyalkylene moieties as antifoamers for acid gas scrubbing processes. For example, U.S. Pat. No. 3,862,243 (in the list above) discloses polyoxyethylene polyoxybutylene block copolymers of the formula R(OEt).sub.n (OBu).sub.m OH where R is an alkyl or alkenyl group having 6-22 carbon atoms, OEt is oxyethylene, OBu is oxybutylene, n is from 3 to 22 and m is from about 3 to 15. It is disclosed in this patent that these block copolymers may be used as antifoamers in alkanolamine acid gas scrubbing processes. Also, the technical brochure entitled "The Wonderful World of Pluronic Polyols", published 1973 by BASF Wyandotte Corporation, Wyandotte, Mich. 48192 discloses polyoxyethylene polyoxypropylene block copolymers as antifoamers and in particular reference is made to their use in "gas-treating units".
It has been recently discovered that high foams in aminecarbonate solutions, where the amine has a large hydrophobic moiety, do not respond adequately to conventional hydrocarbon- and silicone-based antifoaming agents when in use in acid gas scrubbing processes. Most conventional antifoaming agents work by spreading at the air/solution interface thereby disrupting the films formed by surface active materials dissolved in the solution. This mechanism has been found to be ineffective in hot aminecarbonate solutions where the amine has a large hydrophobic moiety in addition to its hydrophilic amine groups. This problem is particularly acute in the case of sterically hindered amine activators in hot carbonate solutions such as disclosed in U.S. Pat. No. 4,094,957 and U.S. Pat. No. 4,112,050.
It has now been discovered that foam-breaking of these hydrophobe-containing amine-carbonate solutions can be accomplished by lowering the hydrophile-lipophile balance (HLB) of the foam-forming surfactants (i.e., the hydrophobe-containing amines act as surfactants in the hot carbonate solutions) by the addition of a low HLB surfactant which is itself a low foamer. It has been found that one class of low HLB surfactants useful as a foam-breaker in these hydrophobic amine-carbonate solutions are the polyoxyethylene polyoxypropylene block copolymers described herein. This discovery is unexpected, since the use of these block copolymers in aqueous amine acid gas scrubbing solutions (i.e., those solutions where the amine is used as the sole absorbent) are ineffective in controlling foaming. For example, it has been shown that the addition of BASF-Wyandotte Pluronic polyol L72 to an aqueous solution of methyldiethanolamine (MDEA) or N-cyclohexyl-1,3-propanediamine (CHPD) gas treating solution caused excessive foaming.